Page 307 - Compression Machinery for Oil and Gas

292 SECTION II Types of Equipment

TABLE 6.3 Optional Instrumentation for Dry Screw Compressors

Alarm low Trip low
In order to prevent In order to prevent
Discharge pressure excessive overcompression excessive overcompression
Pressure difference
between suction and Alarm high Trip high
discharge
If bearings or shafts may be If bearings or shafts may be
overloaded overloaded
p 2 –p 1
Alarm high Trip high
For compressors with For compressors with
varying operating pressure varying operating pressure
levels and if discharge levels and if discharge
Pressure ratio
temperature may become temperature may become
p 2 /p 1 too high too high
Compressor speed for
variable speed drivers Alarm high Trip high
or turbines Alarm low Trip low
Seal liquid
temperature (for oil- Alarm high Trip high
cooled seals or water- Alarm low if liquid may Alarm low if liquid may
cooled seals) freeze or clog freeze or clog
Alarm low Trip low
Seal gas pressure (for
all kinds of seals with Alarm high, if seals can be Trip high, if seals can be
external gas supply) damaged by high pressure damaged by high pressure
Alarm low Trip low
Seal liquid pressure
(for oil-cooled seals or Alarm high, if seals can be Trip high, if seals can be
water-cooled seals) damaged by high pressure damaged by high pressure
Shaft axial thrust Alarm high Trip high

design pressure of the casing is 40bar the maximum allowable differential pres-
sure may be limited to 1.5MPa by the rotors and bearings. Thus the compressor
may be able to compress from 1.5 to 3MPa but not from 1.5 to 4MPa. In this
case a differential pressure monitoring is required to ensure that the compressor
rotors and bearings are not overloaded. Due to the variety of rotor designs and
L/D only very rough values for the maximum allowable differential pressure of
dry screw compressors can be given. This may range from 0.4MPa for long
rotors with 3/5 profile up to approximately 1.5MPa for short rotors with 4/6
profile. The allowable differential pressure must be determined on a case-by-
case basis. Examples for high discharge pressure or high-pressure difference
are no. 3 in Table 6.6 and no. 12 in Table 6.8.